Lowery, et al. U.S. Pat. No. 3,492,479 discloses a portable nuclear gauge that utilizes a fast neutron source and a thermal neutron detector for determining the composition of a bulk material placed in a sample pan. This gauge is particularly useful for measuring the asphalt content of a bituminous paving mix. This type of gauge relies upon the neutron moderating characteristics of hydrogen present in the composition for determining, for example, the amount of asphalt in a paving mix or the amount of moisture in a building material. For these determinations it is known that the amount of asphalt or the amount of moisture can be related to the hydrogen content of the material. The hydrogen content of the material can be determined by subjecting the sample to radiation from a fast neutron source and detecting neutrons that have been slowed or thermalized as a result of interaction with the hydrogen nuclei present in the sample. The number of thermalized neutrons detected and counted over a period of time is utilized in determining the hydrogen content of the sample.
A more recent model of this gauge has been produced by applicant's assignee embodying the principles of the Lowery patent and sold as the "Model 3241 Asphalt Content Gauge" by Troxler Electronic Laboratories, Inc. One such gauge is described in U.S. Pat. No. 4,874,950. This gauge includes a microprocessor to facilitate calibration and computation of the asphalt content of the sample.
A typical procedure for preparing samples for testing involves weighing a sample pan, filling the sample pan carefully, and compacting the samples to a uniform volume and weight. For determining the asphalt content of a bituminous paving mix, this procedure entails filling a sample pan with a loose, well-mixed paving mix and carefully compacting the loose mix so that all the test samples will have the same density.
In the course of manufacturing and applying the bituminous paving mix to form pavement, various measurements and tests of the paving mix are customarily made in addition to measuring asphalt content. Many of these tests and measurements require the use of standardized samples, or sample plugs, which are made according to well-established procedures. One such test that is used to measure the compactability of the mix requires the preparation of standardized samples called Marshall plugs. A known weight of paving mix is placed into a compaction mold and hit a predetermined number of times with a hammer of known weight to form a compacted cylindrical plug, the Marshall plug, of about 4 inches diameter and about 3 inches height, from which compactability is measured.
Since standardized samples, such as Marshall plug samples, are already being prepared by these producers and users, it would be convenient if these same standardized samples could be used in the measurement of asphalt content. By using existing samples and avoiding the need for separately preparing a larger sample, the asphalt content determination could be made more quickly and easily. However, prior attempts to use standardized samples such as Marshall plug samples in the measurement of asphalt content have been unsuccessful. The measurement readings were not of sufficient accuracy and reproducibility to be acceptable. In this regard, several years ago applicant's assignee commissioned a university to investigate how to use Marshall plug samples in the measurement of asphalt content with the Troxler 3241 Asphalt Content Gauge. This study is described in a 1985 engineering report of the Department of Civil Engineering of Clemson University entitled, Asphalt Content Determination of Marshall Specimens by the Nuclear Method. However the test procedure reported in this study was never adopted commercially because it did not provide acceptably accurate and reproducible asphalt content readings.
With the foregoing in mind, it is an object of the present invention to overcome the limitations of the prior practices discussed above and to provide a method and apparatus capable of accurately measuring samples that are already customarily prepared so as to thus avoid the necessity of a separate sample preparation step.